If properly processed, post chlorinated polyvinyl chloride (CPVC) develops useful physical properties in addition to the inherent higher heat deflection temperature (HDT) compared with polyvinyl chloride. CPVC is a starting material for thermoplastic compounds for hot water pipes, and particularly pressure pipes, waste gas pipes, ducts, storage tanks, and other construction and industrial materials among other uses. The aim of the invention pertains to CPVC compounds which have unique combination of preferred properties: a notched izod impact strength of at least about 1.2 ft.multidot.lbf per inch notch (64 J/m of notch), a tensile strength of at least about 7,000 psi (48.25 MPa), a modulus of elasticity of at least about 360,000 psi (2,481 MPa), and an unannealed heat deflection temperature (HDT) under a 264 psi (1.82 Mpa) load of at least 110.degree. C. When formed into an extruded pipe, the compounds have good drop impact and exhibit a long term hydrostatic design strength of at least 750 psi at 200.degree. F.
The practical limits for processing CPVC conventionally, are primarily determined by the chlorine content. Conventional CPVC having 63 to 68% chlorine can be formulated to perform under a hydrostatic design stress at about 80.degree. C., and is generally formulated with impact modifiers and flow enhancing additives among other ingredients to arrive at a balance of processibility and the required physical properties. Additives chosen for processing ease are usually detrimental to development of the best physical properties, due to variation in the extent of fusion occurring in the extrusion melt under mechanical shear forces.
Antagonistic relationships have been observed between the amount of impact modifier and resulting melt flow rate, heat deflection temperature, tensile modulus, dynamic thermal stability, and weatherability. Moreover, flow enhancers or processing aids assist in improved extrusion characteristics but can be detrimental to tensile strength, tensile modulus, ductility, and heat deflection temperature. The present invention pertains to the development of long sought physical properties with CPVC having 69 to 74 weight percent chlorine without endangering the work force nor damaging the processing equipment when being processed. At this chlorine level the processing behavior of CPVC significantly departs from the easier processing types with lower chlorine contents. The problem is not addressed by the simple addition of more stabilizer, or more processing aid. The balance of properties desired herein cannot be achieved in this manner.
For example, it was observed that on processing 69 to 74 percent chlorine CPVC with the use of normal amounts of a diene containing impact modifier resulted in such poor processing stability that the compound began to rapidly degrade within a matter of a few minutes after reaching extrusion conditions. This level of degradation presented a serious health and environmental concern in the processing plant. Thus ABS, MBS MABS, and other conventional impact modifiers containing more than about 10 weight percent polydiene cannot be used in the present invention at amounts exceeding about 2 weight parts per 100 weight parts of CPVC.
It has been found that for pressure pipe, CPE as the sole impact modifier/processing aid at above about 6 weight parts per 100 weight parts (phr) of CPVC having 69-74% chlorine content fails to meet tensile modulus, izod impact per ASTM-D 1784, and long term hydrostatic design strength per ASTM-D2846, or F-441. Also a desired ductility range cannot be achieved for the finished pipe.
There are few references in the literature dealing with compounding CPVC for low shear extrusion such as for the manufacture of plastic piping. U.S. Pat. No. 3,453,347 discloses impact strength improved CPVC on addition of amorphous rubbery polymerized alkylene monoepoxides and CPE. The particular oxirane monomers found to produce a rubbery amorphous polymer contain at least 3 consecutive carbons, for example butene-1-oxide. CPE is present at from 5 to about 10 parts and the polyepoxide is present from about 0.25 parts to about 2.5 parts per hundred weight parts CPVC. The inherent viscosity of the parent PVC from which the chlorinated PVC is derived has a preferred level of greater than about 0.55. The actual examples were with CPVC resin having 65.7% chlorine. Compounds derived from this resin are wholly inadequate in achieving the properties of the present invention.
It was found that in processing 69-74 % chlorine content CPVC with the use of a moderate amount (8 wt. parts or more) of chlorinated polyethylene, the hydrostatic stress rupture properties are not acceptable. It was also found that the degree of chlorination and Mooney viscosity played a critical role in the development of long term hydrostatic stress rupture properties. When more chlorinated polyethylene was used, the stress rupture properties worsened.
Powder compounds in use today are required for high output extrusion processes. Higher output rates are attained with powder compound and using certain expertly operated, larger extrusion machines, output rates have exceeded 900 lbs./hr. This leads to a narrower processing window and places a higher demand on a powder extrusion compound based on CPVC in terms of processibility. In the short amount of residence time under high temperature and controlled shear, a powder compound must achieve complete fusion and resist decomposition in contact with surface temperatures which could otherwise break-down CPVC in a matter of minutes, rendering the fabricated article unsalable. This problem is particularly acute when handling 69-74% chlorine content CPVC since higher processing temperatures are required than for conventional pipe-grade CPVC.
It is not an answer to the problem of achieving higher design stress ratings by using a CPVC having somewhat lower chlorine content plus the use of a Tg enhancing additive. A high strength blend of CPVC and a styrene-acrylonitrile (SAN) copolymer was disclosed for instance in U.S. Pat. No. 4,647,646. The blend exhibits preferably a single homogenous phase wherein the preferred embodiment consists essentially of CPVC having between 60.5% and 64.5% chlorine and styrene-co-acrylonitrile (SAN) containing between 18% and 24% acrylonitrile. The blend exhibits improved tensile strength, however a particularly high tensile strength in the absence of adequate impact resistance is not useful for CPVC pressure pipe such as plumbing pipes and fittings, drain-waste-vent systems, or industrial pressure piping uses. A different combination of properties is required which is much more difficult to achieve. Significant impact modification of this blend would most likely present processing problems, and a loss in HDT and tensile modulus.
An improved melt processible CPVC composition is disclosed in U.S. Pat. No. 4,584,349 ('349) comprising a CPVC polymer having chlorine content of between about 60% and 66% in combination with poly-methylmethacrylate. The blends exhibit a substantially single phase morphology and the glass transition temperature (Tg) of the blends was higher than the Tg for the CPVC and PMMA separately. Tg for amorphous polymers is highly correlated with HDT. In some applications, achieving a HDT higher than CPVC is desirable. Higher Tg and improved melt flow are achieved in '349, however compositions with high HDT and melt flow alone are not entirely acceptable for pressure pipe applications without additional properties, particularly, stress rupture properties, tensile strength, tensile modulus, and impact properties achieved from the direct extrusion from powder.
U.S. Pat. No. 4,710,533 ('533) discloses CPVC blends comprising a combination of a flow enhancing amount of alpha-methyl styrene polymers and/or alpha-methyl styrene-co-acrylonitrile, an impact modifier of ABS or MBS, lead stabilizer(s), acrylic processing aid and lubricant(s). The blends exhibit good thermal stability, impact strength and melt viscosity, however an HDT of no higher than 91.5.degree. C. is achieved. HDT is a critical property for CPVC pressure pipe and a minimum value of 110.degree. C. is required for qualification under cell class 2-3-4-4-8. In addition, unacceptable impact performance and tensile modulus for the compositions of '533 would be expected. The further addition of impact modifier might improve somewhat the impact strength but would not correct the deficiency in HDT, and would likely lower HDT in addition to decreasing processibility.
Thus, processing of higher chlorine content CPVC presents new problems and attempts to balance all of the desired properties particularly for powder extrusion processing are met with sacrifices in at least one property such as HDT or tensile strength when pursuing improvements in another property. There is considerable art and less science demonstrated both for formulating a composition and in processing that composition to develop all the required properties for improved higher temperature uses for CPVC pipe compounds.
Applications for industrial uses such as steam condensate lines have heretofore been beyond the capabilities of conventional CPVC pipe under these tolerance levels. Accordingly, it would be desirable to obtain a CPVC composition which in the fused state has an HDT of at least 240.degree. F. (115.degree. C.) and approaches or meets the requirements of cell class 2-3-4-4-8 or higher, and it would be desirable to provide a pipe therefrom exhibiting a minimum 200.degree. F. (93.degree. C.) hydrostatic design stress of at least 375 psi (2.58 MPa) or higher per ASTM-D2837, while maintaining adequate ductility. Such a composition and article derived therefrom which exhibits this desired combination of properties has not been heretofore disclosed and represents a long felt need in the art pertaining to CPVC pressure pipe components for higher use temperature service, such as the a forementioned industrial piping systems especially in contact with materials such as steam. With regard to this balance of properties it would be preferable, moreover a practical necessity, for achieving this combination in an extruded or molded article directly from a powder compound. The compound must also exhibit adequate dynamic thermal stability for use in commercial extrusion and injection molding processes.